For CT services, utilization "more than doubled" with self-referrals, while non-self-referred CTs increased about 30%, the study found.

With the findings, GAO estimated that providers who self-referred ordered about 400,000 more advanced imaging services than they would have if they weren't self-referring, which cost Medicare an additional $109 million in 2010.

"To the extent that these additional referrals were unnecessary, they pose unacceptable risks for beneficiaries, particularly in the case of CT services, which involve the use of ionizing radiation that has been linked to an increased risk of developing cancer," GAO said.